1. Field of the Invention
The invention relates to inverters for supplying power to motors and, in particular, to two or more balanced or matched inverters for supplying power to AC traction motors, particularly traction motors of non-highway vehicles.
2. Description of the Prior Art
Off highway vehicles with electric propulsion used for heavy duty are typically equipped with DC power source such as a diesel engine supplying power to an alternator. The output of the alternator is rectified to form a DC link which supplies power to a variable frequency, variable voltage inverter. The inverter drives a three phase induction motor connected to the rear wheels of the vehicle. As the requirements for haulage capability of such vehicles increases, a corresponding increase in the power required to propel such vehicles is needed. This in turn increases the power rating requirements of the semiconductors which provide the switching of the variable frequency, variable voltage inverter.
FIG. 1A illustrates a typical prior art three phase single inverter including a DC bus and six electronic switches which are selectively opened and closed to feed power to a three phase AC motor. The electronic switches S1–S6 consist typically of IGBT or GTO switches and their associated diode and snubber components.
FIG. 1B illustrates a prior art timing diagram of the individual commands for six step operation of the six switches S1–S6 illustrated in FIG. 1A. This timing diagram is referred to as an “ideal” timing diagram because the timing illustrated assumes instantaneous switching of the switches so that the switches transition from on to off or off to on instantaneous. As will be noted below with regard to the invention, in practice such switches have timing delays and such timing delays can be problematic if not controlled and/or compensated. In the square wave or six step operation illustrated in FIG. 1B, the six electronic switches S1–S6 are turned on every 60° in the proper order to produce a three phase balanced waveform. For example, when switch S1 is commanded ON, its IGBT is gated on ON and the current will flow through the IGBT or through its anti-parallel diode. If the current is positive (into the AC traction motor), then the IGBT of S1 will be conducting. If the current is negative (from the motor), then the anti-parallel diode connected to the IGBT of S1 will conduct. In either case, the switch S1 which is commanded will be conducted.
FIG. 1C illustrates prior art phase commands for pulse width modulated (PWM) operation of the inverter of FIG. 1A. PWM is used to vary the power supplied to the AC motor. During PWM operation, the six electronic switches S1–S6 are turned ON and OFF at a much higher switching frequency than the fundamental frequency of ON and OFF operation as illustrated in FIG. 1B. In FIG. 1C, the exemplary commands that may go to one of the switches is illustrated. The commands provided to its corresponding switch in the same phase would be opposite the commands illustrated in FIG. 1C, except for minimum ON/OFF and snubber reset times required. For example, when the top switch S1, S3, S5 is ON, its corresponding bottom switch S4, S6, S2, respectively, is OFF and vice versa. FIG. 1C shows firing pulses for a given phase. Other phases are delayed by 120 degrees and 240 degrees.
FIG. 2 illustrates a prior art block diagram of a control logic for a single inverter. The traction motor control logic illustrated in FIG. 2 generates the firing commands f4 and f5 to phase A. The frequency and timings are controlled such that the AC motor generates the desired amount of torque. Thus, the control logic varies the flux, frequency, voltage, current etc in the machine. The control logic also functions to protect the traction motor and the traction inverter components. The output of this logic is a three phase command, one to each of the phases A, B and C. When a firing command signal is high, its corresponding top (positive) switch is turned ON and when the signal is low the bottom (negative) switch is turned ON. Signal f1 is the phase A command signal so that FIG. 2 illustrates the details of phase A. Phases B and C are similarly configured. The phase A firing command is split into the top and bottom switch command signals f2 and f2. The gate driver/switch receives signals f2 and f3 and the status feedback is sent back to the control logic. Signals f4 and f5 are the status feedback from the top and bottom switches. Operation of phase B and C are similar.
FIG. 3 is a prior art timing diagram of the logic commands. This figure illustrates the timing diagrams of the various signals for phase A described in FIG. 2. At time t0, the phase A command signal f1 transitions from 0 to 1 indicating that the top switch of phase A should be ON and the bottom switch should be OFF. Since previously the bottom switch was ON, the first event is at t1 where switch command signal f3 (which is the command to the bottom switch) goes low, commanding the bottom switch to turn OFF. At time t2, the bottom switch turns OFF and the status feedback signal f5 transitions to a low value indicating such turn off. This f5 transition is detected by the phase A firing command logic which then commands the top switch ON at time t3 as indicated by the switch command signal f2 going high. This occurs after a short period of time to allow any snubber settling time or margin. As a result, the status feedback signal f4 transitions to a high value at t4 indicating the top switch is ON. This completes a 0 to 1 transition of a phase A command signal f1. Similar timings are illustrated in FIG. 3 for a transition from 1 to 0. In particular, at t5, the phase A command signal f1 transitions to 0, at t6 the top switch command signal f2 transitions to OFF, at t7 the top switch status feedback signal f4 transitions to OFF, at t8 the bottom switch command signal f3 transitions to ON and at t9 the bottom switch status feedback signal f5 transitions to ON. This complete cycle is repeated again starting at time t10.
Thus, as illustrated in FIG. 1A wherein a single inverter supplies all power to an AC motor, it is apparent that the amount of power supplied to the AC traction motor is controlled by and related to the amount of power that can be supplied through a single switch of each phase of the single inverter. As a result, the total power that can be supplied to the AC traction motor is limited by the power rating or the maximum power that each switch can handle. In order to increase the load bearing capacity of a non-highway vehicle, for example, an off highway vehicle, a freight locomotive or a passenger locomotive, the AC traction motor and the power supply to it must be increased to provide more torque for handling the additional load. This in turn requires an increase in the power handled by each of the switches. Unfortunately, IGBTs or GTOs have a limited power handling capability and the cost of designing and manufacturing higher power switches can be prohibitive. Therefore, there is a need for an inverter for supplying power to AC traction motors which can handle additional power requirements and which has a reasonable manufacturing and replacement cost. There is also a need for employing similar components in such inverters as the components presently in use in existing non-highway vehicles in order to reduce the inventory necessary to supply spare parts for such vehicles. There is also a need for a multiple inverter configuration for supplying power to traction motors in which the inverters are balance or matched so that the inverters operate with disparity such that one inverter carries significantly more current than the other, which could cause overheading or burnout.